Google originally began selling Glass Explorer units at I/O 2012, giving developers in attendance the opportunity to be the first to get a pair of its smart eyewear so long as they were willing to part with $1500. More recently, El Goog began allowing existing Explorers to invite their friends to take part in the testing, and now Google is the one doing the inviting.

Google today began sending “VIP invitations” to Play Music All Access subscribers that give them the chance to purchase a Glass unit. The invitations are meant to coincide with the recent arrival of Play Music support on Glass, which allows wearers to listen to their playlists and radio.

While the Glass Explorer program still isn’t wide open for anyone to join, this latest round of invitations that’s being sent to Play Music All Access subscribers is a pretty large expansion of the program. It’s also an indicator that Glass is continuing to mature and get closer to being ready for a full-on public launch, which Google has previously said will take place in 2014.

Have you received an invitation to join the Google Glass Explorer program?